1. Field of the Invention
The present invention relates generally to telephony, and more particularly to tone detection.
2. Related Art
Among analog telephones the most common way to transmit dialing information is by sending pairs of sinusoidal tones at relatively high signal levels. This technique is known as dual-tone multiple frequency (DTMF) and is also commonly called TouchTone (formerly a trademark of AT&T). Telephones and other equipment that use DTMF tones must assert a pair of tones at specific prescribed frequencies and signal levels in order to communicate digits. Other equipment in a telephone network that must be receptive to such dialing information typically incorporates a tone detector or DTMF detector.
The DTMF signaling convention is designed to reliably convey dialed digits while preventing spurious detection of non-dialing signals, such as normal speech during a telephone conversation. The DTMF detectors are intended to be very robust. In order for a signal to qualify as a DTMF tone, there must be exactly two strong sinusoidal signals present--one from a low-frequency group (697, 770, 852 or 941 Hz) and one from a high-frequency group (1209, 1336, 1477 or 1633 Hz). As transmitted, each frequency must be within 1.5% of its nominal frequency, and must persist steadily for at least 50 ms. As received by a tone detector, each frequency must be within 3.5% of its nominal frequency, and must persist steadily for at least 25 ms. In addition, the signal level received at the tone detector must exceed -55 dBm. The frequency plan is carefully chosen so that harmonics and intermodulation products are unlikely to emulate any of the other possible valid tone combinations, and is documented in Bellcore BOC Notes on the LEC Networks 1994, SR-TSV-002275, Issue 2, April 1994, paragraph 6.13.2.
A tone detector may use even more sophisticated techniques to further ensure that the two tones presented are the principal components of the signal energy. In most cases, a speech signal will occupy a broad spectrum. A spectral analysis or signal-to-noise analysis should be able to readily distinguish a voice from a pair of pure tones.
Despite all of the measures taken to avoid spurious triggering of DTMF detectors by voice signals, there are instances where a particular telephone user's voice often satisfies a DTMF detector. During a telephone call, this user is frequently disconnected or unintentionally diverted to another calling feature. For example, this user's speech may emulate the tones generated by pressing the "#" key on the keypad of a TouchTone telephone, causing the network to disconnect the call and give the user a dial tone in expectation of further dialing instructions. These users are uniquely plagued with the frustration of using a telephone network that handles their voice unreliably. What is needed is a way to prevent voice signals from triggering tone detectors.